On Mon, Jan 18, 2010 at 4:37 PM, Poul-Henning Kamp <[email protected]>wrote:
> In message <[email protected]>, "Michael > S. Fis > cher" writes: > >On Jan 18, 2010, at 5:20 AM, Tollef Fog Heen wrote: > > >> My suggestion is to also look at Cache-control: no-cache, possibly also > >> private and no-store and obey those. > > > >Why wasn't it doing it all along? > > Because we wanted to give the backend a chance to tell Varnish one > thing with respect to caching, and the client another. > > I'm not saying we hit the right decision, and welcome any consistent, > easily explainable policy you guys can agree on. Well, the problem is that application engineers who understand what that header does have a reasonable expectation that the caches will obey them, and so I think Vanish should honor them as Squid does. Otherwise surprising results will occur when the caching platform is changed. Cache-Control: private certainly meets the goal you stated, at least insofar as making Varnish behave differently than the client -- it states that the client can cache, but Varnish (as an intermediate cache) cannot. I assume, however, that some engineers want a way to do the opposite - to inform Varnish that it can cache, but inform the client that it cannot. Ordinarily I'd think this is not a very good idea, since you almost always want to keep the cached copy as close to the user as possible. But I guess there are some circumstances where an engineer would want to preload a cache with prerendered data that is expensive to generate, and, also asynchronously force updates by flushing stale objects with a PURGE or equivalent. In that case the cache TTL would be very high, but not necessarily meaningful. I'm not sure it makes sense to extend the Cache-Control: header here, because there could be secondary intermediate caches downstream that are not under the engineer's control; so we need a way to inform only authorized intermediate caches that they should cache the response with the specified TTL. One way I've seen to accomplish this goal is to inject a custom header in the response, but we need to ensure it is either encrypted (so that non-authorized caches can't see it -- but this could be costly in terms of CPU) or removed by the last authorized intermediate cache as the response is passed back downstream. --Michael
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